1. Field of the Invention
The present invention relates to a variable displacement pump used in a power steering apparatus for a motor vehicle or the like.
2. Description of the Related Art
Conventionally, in order to assist a steering force by a hydraulic power steering apparatus for a motor vehicle, a variable displacement pump described in Japanese Patent Application Laid-Open (JP-A) No. 8-200239 is used. This conventional variable displacement pump is directly rotated and driven by an engine of the motor vehicle. The variable displacement pump has a rotor within a cam ring moveably and displaceably fitted to an adapter ring fitting to a pump casing, thereby forming a pump chamber between the cam ring and the outer peripheral portion of the rotor.
In this conventional art, the cam ring is structured to be movable and displaceable within the adapter ring. An urging force maximizing the capacity of the pump chamber is applied to the cam ring by a spring. The first and second fluid pressure chambers are separately formed between the cam ring and the adapter ring. A switch valve controls the fluid pressure supplied to both of the fluid pressure chambers. This corresponds with the amount of discharge flow of a pressurized fluid from the pump chamber so as to move the cam ring. The capacity of the pump chamber is changed so as to control the discharge flow amount from the pump chamber. Accordingly, in this variable displacement pump, the amount of discharge flow is controlled to be large so as to obtain a large steering assist force when the motor vehicle stops or runs at a low speed, where the motor vehicle has a low rotational speed. The discharge flow amount is controlled to be equal to or less than a fixed amount, making the steering assist force small when the motor vehicle runs at a high speed, where the motor vehicle has a high rotational speed, whereby it is able to generate the steering assist force required for the power steering apparatus.
In the conventional art, a direct-drive type relief valve is provided in a pump discharge side passage so as to relieve the fluid pressure when the fluid pressure in the pump discharge side becomes too large due to a static turn steering state in the power steering apparatus being maintained or the like.
Since the relief valve placed in the pump discharge side passage in the conventional art is the direct-drive type, a change of relief pressure in accordance with a passing flow amount (a pressure override characteristic) is large. The passing flow amount tends to increase in accordance with an increase in the rotational speed, and reduce due to a reduction of an oil temperature. Accordingly, in a variable displacement pump with a direct-drive type relief valve in accordance with the conventional art, the used rotational speed and the oil temperature change affect it, making it impossible to obtain an inherently required relief pressure.
An object of the present invention is to make it possible to set a stable relief pressure even when using conditions (a rotational speed and an oil temperature) are changed, when relieving an excessive fluid pressure in a pump discharge side, in a variable displacement pump.
In accordance with the present invention, there is a variable displacement pump comprised of a rotor rotated and driven while fixed to a pump shaft inserted to a pump casing and receiving a multiplicity of vanes in a groove so as to be movable in a radial direction. It contains an adapter ring fitted to a fitting hole in the pump casing and a cam ring fitted to the adapter ring so as to form a pump chamber between the cam ring and an outer peripheral portion of the rotor. The cam ring is movable and displaceable within the adapter ring and separately forms the first and second fluid pressure chambers between the cam ring and the adapter ring. A switch valve operated due to a pressure difference between an upstream side and a downstream side in a metering orifice provided in the pump discharge side passage controls the supply of fluid pressure to the first and second fluid pressure chambers in correspondence to a discharge flow amount of a pressurized fluid discharged from the pump chamber. This moves the cam ring and changes the capacity of the pump chamber, thereby making it possible to control the discharge flow amount discharged from the pump chamber. A relief valve relieves the excessive fluid pressure in the pump discharge side. The relief valve is constituted by a pilot drive type relief valve obtained by adding a pilot valve to a main valve. The fluid pressure in the downstream side of the metering orifice provided in the pump discharge side passage is applied to the pilot valve, and the main valve is capable of opening and closing the downstream side passage of the metering orifice with respect to a drain passage.
In accordance with the present invention, there is a variable displacement pump comprising a rotor rotated and driven while fixed to a pump shaft inserted to a pump casing and receiving a multiplicity of vanes in a groove so as to be movable in a radial direction.
It contains an adapter ring fitted to a fitting hole in the pump casing and a cam ring fitted to the adapter ring so as to form a pump, chamber between the cam ring and an outer peripheral portion of the rotor. The cam ring is movable and displaceable within the adapter ring and separately forms the first and second fluid pressure chambers between the cam ring and the adapter ring. A switch valve operated due to a pressure difference between an upstream side and a downstream side in a metering orifice provided in a pump discharge side passage controls the supply of fluid pressure to the first and second fluid pressure chambers in correspondence to a discharge flow amount of a pressurized fluid discharged from the pump chamber. This moves the cam ring and changes the capacity of the pump chamber, thereby making it possible to control the discharge flow amount discharged from the pump chamber. A relief valve relieves the excessive fluid pressure in the pump discharge side. The relief valve is constituted by a pilot drive type relief valve obtained by adding a pilot valve to a main valve. The fluid pressure in the upstream side, of the metering orifice provided in the pump discharge side passage is applied to the pilot valve, and the main valve is capable of opening and closing the upstream side passage of the metering orifice with respect to a drain passage.